Conventional electric vehicles are known to those skilled in the art to be equipped with a high voltage traction battery for use as a primary source of power and an auxiliary battery for use in powering various electrical components. For obvious design reasons, the high voltage traction battery is typically isolated from the vehicle chassis to prevent the formation of a current path when a person concurrently contacts the vehicle and one of the traction battery terminals. Even in this arrangement, however, it is still possible to cause a short circuit between the traction battery and the body of the vehicle due to the potential leakage of electrolyte from the battery or for other reasons. If such leakage does occur, a resistive path is developed through which leakage current may flow.
Previous work in detecting resistive paths of the type referenced above have involved deliberately providing a return path for current to flow through the resistive path into the vehicle chassis and back to the traction battery. Unfortunately, these designs create the very resistance path that the test is intended to detect. Thus, if a person concurrently touches the vehicle and the traction battery during the test, a resistive path may develop, and result in the unwanted flow of current. The other disadvantage known to those skilled in the art is that the mechanical contacts historically used in prior art leakage path detection devices become dirty and require regular servicing.
See, for example, U.S. Pat. No. 3,696,367 issued to Inamdar on Oct. 3, 1972. Inamdar discloses a ground fault detection circuit for use with dual voltage battery systems, comprising first and second ground connected circuits which are alternatively energized depending on the location of a ground fault in any one of three lines connected to the system. Those skilled in the art will recognize that Inamdar physically connects the high voltage and low voltage batteries used by the electric vehicle.
See, also, U.S. Pat. No. 3,786,466 issued to Naito, et al., on Jan. 15, 1974. Naito discloses a leakage detecting circuit for detecting leakage from a source of power, such as a battery, to a source of reference potential. The circuit comprises a pair of voltage detecting circuits respectively connected to the positive and negative terminals of the battery to ground through first and second switching means. As discloses by Naito, the battery is insulated from ground so that if a leakage path exists between the battery and ground, an electrical circuit is closed which includes a voltage detection device and one of the switching means. Conventional electrical leakage detecting devices such as those referenced above have proven inoperable due to their inability to detect leakage current without creating the very leakage situation for which they are designed to detect.